When printing security documents, in particular banknotes, care must be taken to monitor the production carefully. An unaccounted loss of documents during the printing process has to be avoided.
It has been known to monitor the sheets that the security documents are printed on. For this purpose, The sheets are provided with machine readable identifiers, which are checked by the various printing stations of a security printing plant. This allows to detect a loss of a sheet and to find the location where it was lost. This solution requires, however, a substantial amount of hardware, and in particular a powerful, dedicated network for exchanging the substantial amount data that is generated in a large printing plant with high speed printers.